It is often desirable in digital systems and particularly in digital data processing systems to measure the rate of occurrence and/or duration of occurrence of digital events. In this context a digital event is sensed as a binary state of true or false within a subject conductor. If one considers the rate of occurrence and duration of occurrence together as integrated over time, the measure may be termed duty cycle. The measurement of duty cycle of a digital event is as satisfactory as separate measurements of rate of occurrence and duration of occurrence when only a relative measurement is needed and either rate of occurrence or duration of occurrence is fixed or well known. Furthermore, in many cases, measurements of rate of occurrence and duration of occurrence are desirable only to compute duty cycle.
Rate of occurrence, duration of occurrence, and duty cycle of digital events may all be measured directly. This normally involves measuring in a fashion which is synchronous to occurrence of the digital events. For digital data processing systems this has taken the form of software measurement. This technique has two basic disadvantages. First, to be measured, a digital event must be capable of being sensed by software. This sensing is, of course, indirect and is usually limited to digital events which are synchronous to execution of computer instructions. The second disadvantage of software measurement is the time penalty to be paid. The execution of instructions to perform the measurements may consume a considerable portion of the digital data processing system's capacity.
An alternate method of synchronously measuring rate of occurrence and duration of occurrence of digital events is through use of special purpose dedicated hardware. This method is currently most popular because of the disadvantage of software measurement as mentioned above. This method finds its greatest applicability to digital events whose rate of occurrence is relatively small in relation to the measurement period and whose duration of occurrence is either very long or very short relative to the measurement period. Whereas these restrictions are not intrinsically required by the measurement method, the amount of hardware required to perform these measurements is usually large enough to force these restrictions. A further disadvantage of synchronously measuring rate of occurrence and duration of occurrence is that integration of these quantities over the measurement period is required if measurement of duty cycle is desired.
Ross D. Rash, et al, in U.S. Pat. No. 3,588,837, disclose a method of measuring digital events based upon sampling. Hardware in the Rash Patent senses or samples the binary state of the subject conductor at times intended to be random to the expected occurrence of the digital event. By sampling the subject conductor a statistically significant number of times, a measure of duty cycle is obtained. The confidence level of that measurement is easily determined using techniques of elementary statistics. The reader might find review of the subject helpful through the use of a text such as, Morris Hamburg, Basic Statistics, (Harcourt, Brace, Jovanovich, Inc., 1974).
The present invention also utilizes a sampling technique to derive the measure of duty cycle. Unlike the Rash Patent, however, the present invention does not rely upon the use of an asynchronous sampling clock to provide samples taken randomly in time. The present invention incorporates a random number table stored within a programmable read only memory (PROM) to ensure randomness and correspondingly reduce the error resulting from such lack of randomness. Also, unlike the Rash Patent, the present invention samples multiple subject conductors simultaneously to overcome the speed disadvantages of the sequential sampling in Rash. This permits measurement of rate of simultaneity of digital events in multiple subject conductors.